1. Field of the Invention
The present invention relates to a supported catalyst in the form of cylindrical granules, for 1,2-dichloroethane synthesis by fixed-bed ethylene oxychlorination. More particularly, the present invention relates to a catalyst comprising CuCl.sub.2 as its active compound, supported on an alumina carrier. The cylindrical granules of the catalyst display a cross-section with at least three points of contact with a circumscribed circumference.
2. Description of the Prior Art
It is known that the synthesis of 1,2-dichloroethane by oxidative chlorination of ethylene can be carried out in a fluidized-bed or fixed-bed reactor. In the first case, a more uniform temperature distribution is obtained inside the reactor (with the arising of localized overheating phenomena--"hot spots"--being prevented); unfortunately, at the expense of a certain difficulty of fluidization due to a tendency of the catalytic particles towards sticking. In the second case, controling the reaction parameters is simplier but due to the reduced heat exchange coefficient between the catalyst granules, and between the latter and the reaction gas, hot spots may appear. The formation of such hot spots should be prevented for reasons of selectivity and to prolong the useful life of the catalyst.
A first attempt to solve the problem of the heat exchange between the granules of catalyst for ethylene oxychlorination had been made by resorting to ring-shaped granules or circular-cylindrical granules having a determined height-diameter ("aspect") ratio. Such type catalyst types are disclosed, in EP-B-54674 and U.S. Pat. No. 4,753,914, for example.
The problem of the heat exchange coefficient is not the only technical problem to be solved in the case of an efficient synthesis of 1,2-dichloethane in a fixed-bed reactor. In fact, a granular catalyst used in fixed-bed ethylene oxychlorination is also required to display the following characteristics:
low resistance to gas flow (low pressure drop with the height of the catalytic bed being the same); PA1 large actual specific surface area, i.e., high surface:volume ratio; and PA1 good mechanical strength in order to prevent the catalytic particles from undergoing breakage as a consequence of bed packing.
The catalysts normally used in a fixed-bed oxychlorination process (which have a spherical, solid-cylindrical, or ring shape, with different dimensions) do not solve the problems discussed above to a satisfactory extent. Furthermore, when catalysts are used which have those shapes that are known from the prior art, the diffusion of the reactant gases inside the interior of the catalyst granules and the back-diffusion of the reaction products from the interior of said granules have often led to very limited results. This means that, when one takes into consideration that the oxychlorination reaction takes place in the heterogeneous system more easily and selectively on the outer surface of the catalyst granule, the oxychlorination catalysts having the shapes known from the prior art are not efficiently used. Therefore, in order to obtain the desired conversion rate, a large amount of catalyst must be used. In the case of a tube-bundle fixed bed, the need for a large amount of catalyst requires the use of tubes having heights suitable to accomodate the large amount of catalyst. With the known forms of oxychlorination catalysts, this causes a further increase in pressure drop, because the empty spaces between the catalyst granules are also limited.
Furthermore, in exothermic reactions, which generate large amounts of heat--such as oxidative, halogenation reactions--a high value of coefficient of heat exchange between the catalyst and the gas feed is required, in order to secure an adequate removal of reaction heat. Adequate removal of reaction heat is necessary to avoid consequent excessive overheating of the catalytic bed, which often damages the catalyst in the bed and/or reduces the catalytic performance thereof. In the case of the catalysts having the traditional shape, in order to accomplish a high heat exchange coefficient, the turbulence of the reaction gas on catalyst particles had to be increased, but such a contrivance would cause a further increase in pressure drops, with consequent increases in operation costs.
Catalysts with shapes different from those traditionally used are disclosed in U.S. Pat. No. 4,441,990; which relates to tubular extruded granules having an essentially triangular or quadrangular, multi-lobed cross-section. With such catalysts, some advantages are achieved in terms of breakage strength and pressure drop. However, the results are not very much different from the results which are obtainable with the traditionally shaped catalysts. Other non-traditional shapes of catalyst granules are disclosed in EP-A-464,633, with specific reference to the process for producing unsaturated esters.
There is a need for a catalyst granule having a configuration, which makes it possible for oxidative halogenation reaction results to be considerably improved.